1. Field of the Invention
The present invention relates to an outboard motor comprising a multi-cylinder engine installed in an engine cover, and, more particularly, to an outboard motor having an efficient supply-and-exhaust structure and fly-wheel magneto structure in an engine compartment.
2. Description of the Related Art
There are prior-art outboard motors, mounted to a transom of the hull of a ship, comprising a V-type multi-cylinder engine or an in-line multi-cylinder engine.
Examples of related prior-art outboard motors are disclosed in Japanese Unexamined Patent Application Publication No. 2002-137792 (JP '792) and Japanese Unexamined Patent Application Publication No. 11-198893 (JP '893).
As shown in FIG. 20, in the outboard motor disclosed in JP '792, a V-type multi-cylinder engine b is disposed in an engine cover a, and the upper portion of an engine compartment c in the engine cover a is partitioned by a partition plate d into a front and a rear portion. In addition, an engine inlet space (path) e is disposed behind the partition plate d. The lower portion of the engine inlet space e opens into the engine compartment c, and, thus, the engine inlet space e is not formed independently of the engine compartment c which includes heat-generating sources such as electrical components and the V-type multi-cylinder engine b.
Also, a forcible-supply-and-exhaust structure is not used in the engine compartment c in the engine cover a. Therefore, heat dissipated from the heat-generating sources, such as the V-type multi-cylinder engine b, in the engine compartment c tends to accumulate. Therefore, a large temperature rise occurs in the engine compartment c. The large temperature rise may overheat the parts in the engine compartment c.
Since the engine inlet space (path) e of engine inlet f opens into the engine compartment c, outside air that flows into the engine cover a is directly affected by heat dissipation caused by combustion in the multi-cylinder engine b and hot lubricant oil circulating in the multi-cylinder engine b. Therefore, the temperature rises. Since air whose temperature has risen in the engine compartment c is used as combustion air, air density is reduced. Consequently, engine output is reduced.
Even in the outboard motor disclosed in JP '893, outside air that flows into an engine cover from the back portion of the top portion of the engine cover flows into an engine compartment through an engine inlet space (path), and is supplied to an engine air-inlet system.
In this case also, since the engine inlet space communicates with the engine compartment from the front inner side of the engine cover, and opens into the engine compartment, air whose temperature has risen in the engine compartment is guided to the engine air-inlet system of a multi-cylinder engine. Therefore, the outboard motor has the same problems as the outboard motor disclosed in JP '792.
Other examples ofl related prior-art outboard motors are disclosed in Japanese Unexamined Patent Application Publication No. 2000-328952 (JP '952), Japanese Unexamined Patent Application Publication No. 10-339167 (JP '167), and Japanese Unexamined Patent Application Publication No. 2001-158397 (JP '397).
In above referenced prior art outboard motors, a vertical multi-cylinder engine is accommodated in an engine cover, and a vertically extending crank shaft (vertical crank shaft) is disposed in the multi-cylinder engine. Since even an outboard motor comprising a vertical multi-cylinder engine needs to be mounted to the transom of the hull of a ship, it is required to be compact and light, and to have its center of gravity disposed at a low position.
As shown in FIG. 21, in such related prior-art outboard motors, a vertical crank shaft h of a multi-cylinder engine g penetrates through the top portion of a crank case i and protrudes upward therefrom by a large amount, and a fly-wheel magneto device j is disposed at this upwardly protruding portion (an upwardly protruding portion n). The fly-wheel magneto device j comprises a fly-wheel k having a large inertia, and a magneto device 1 for generating electrical power.
In order for the fly-wheel k to be stably and integrally rotatably mounted to the vertical crank shaft h, a cylindrical boss m is formed at the central portion of the fly-wheel k, and the upwardly protruding portion n of the crank shaft h is tapered. In addition, a tapered boss hole o of the boss m of the fly-wheel k is externally fitted to the upwardly protruding portion n of the crank shaft h. The hole o and the upwardly protruding portion n are locked by a locking key p, and fastened together by a fastening bolt q.
Although the fly-wheel k, which is fitted to the tapered upwardly protruding portion n of the vertical crank shaft h, is stably and rotationally supported thereby, the contact area is large because the boss m of the fly wheel k that is fitted to the tapered portion of the vertical crank shaft h is thick. Therefore, the axial length of the tapered fitting portion needs to be large.
Consequently, the amount of upward protrusion of the vertical crank shaft h from the crank case i is large, and, thus, the fly-wheel k needs to be tall. For this reason, the position of the center of gravity of the fly-wheel k is high, thereby making it difficult for each of the disclosed outboard motors to have its center of gravity disposed at a low position and to be lightened.
The fly-wheel k needs to have the boss m that does not contribute to the inertia (moment of inertia) of the fly-wheel k. In addition, in order to smoothly rotate the fly-wheel k when the fly-wheel k has the thick boss m, the fly-wheel k needs to have an axial length that is equal to or greater than a certain length. Therefore, the fly-wheel becomes heavier. As a result, it is difficult to make the fly-wheel k light and compact, and the position of the center of gravity of the fly-wheel k is high, thereby hindering stable rotation of the fly-wheel k.
Since the fly-wheel k, which is covered by a fly-wheel magneto cover r, is tall overall, an effectively used space cannot be provided between the fly-wheel k and the fly-wheel magneto cover r.
The fly-wheel k, mounted to the protruding portion at the top end of the vertical crank shaft h, is a big factor in determining the overall height and weight of the multi-cylinder engine g. However, since the fly-wheel k is tall overall, it is difficult for each of the disclosed outboard motors to be compact and light and to have its center of gravity disposed at a low position.
In the outboard motors disclosed in JP '792 and JP '893, the inlet path in the engine cover is not separately formed from the engine compartment which includes the heat-generating sources. Therefore, the engine inlet path opens into the engine compartment. In addition, a forcible-supply-and-exhaust structure is not used in the engine compartment.
For this reason, air warmed in the engine compartment is guided to the engine air-inlet system, as a result of which air density is reduced, thereby reducing engine output and preventing heat in the engine compartment from being effectively exhausted. Therefore, overheating of the parts in the engine compartment is not sufficiently prevented. Consequently, for example, the operation of the parts in the engine compartment, such as electrical parts, is impaired, and it is difficult to ensure durability of the parts.
In the outboard motors disclosed in JP '952, JP '167, and JP '397, the fly-wheel is tall overall, and thus, is heavier. Therefore, the fly-wheel cannot be made lighter and the position of its center of gravity cannot be lowered, as a result of which it is difficult to make the outboard motors compact and light.
Since a boss that does not contribute to the inertia of the fly-wheel is formed in the fly-wheel, the fly-wheel is heavier, and the distance to the center of gravity of the fly-wheel from the top end supporting portion (bearing) of the crank shaft is increased. The larger the distance to the center of gravity of the fly-wheel from the top-end supporting portion of the crank shaft due to the position of the center of gravity of the fly-wheel being high, there is a greater chance that slight variations in rotational balance of the fly-wheel increase vibration. Therefore, a load is exerted upon the crank shaft and bearing more than is necessary, thereby increasing vibration of the crank shaft, impairing durability of the crank shaft, and damaging the crank shaft.